1. Field of the Invention
The invention concerns a pull element travel sensor and a lifting apparatus embodied with such elements.
2. Background of the Invention
Pull element travel sensors occur in many different configurations for example in order to determine the precise positioning of a given component which in particular can move over considerable travel distances, for example the cabin of an elevator. A typical pull element travel sensor of that kind has a tensile or pull element, for example a cable, which is wound on to a winding drum which is biassed in the winding-on direction. The free end of the pull element is connected to that component whose position is to be determined, for example as mentioned above an elevator cabin. A prestressing force for urging the winding drum in the winding-on direction is produced for example by way of a flat spiral spring which for example is arranged coaxially with respect to the winding drum and is non-rotatably connected thereto.
The winding drum is also typically connected to a detection unit for recording the revolutions or angular distances through which the winding drum passes in the winding-on or unwinding direction, and which in addition by way of a suitable electronic evaluation system determines the length of the pull element which has been drawn off the cable drum.
In order to simplify that operation of determining the drawn-off length of the pull element, for example a cable, the pull element is wound in only a single layer on the periphery of the winding drum, with the turns of the pull element on the winding drum being in axially juxtaposed relationship. As a result, one revolution of the winding drum will always correspond to exactly the same length of the pull element.
In order to ensure that the pull element is wound on to the drum in only a single layer and in order to prevent the pull element from jumping over on to the first layer to start a second layer there over when winding the pull element on to the winding drum, it is generally necessary to take suitable mechanical steps involving appropriate structure on the travel sensor to obviate that occurrence.
Instead of determining the rotary movement of the winding drum, it is also known for the length of the pull element, which has been drawn off the winding drum, to be determined directly, for example by arranging a counting device at the location at which the pull element issues from the housing of the winding drum, wherein the pull element has portions which are always of the same size and which are of different configurations, for example in the form of holes or perforations which are arranged uniformly in the longitudinal direction. Those different portions such as holes can be suitably counted by the counting device and converted in a suitable system into a measurement corresponding to the length of the pull element which has been drawn off the winding drum.
A disadvantage with that structure is that it is necessary to use a pull element of a special configuration whose differing portions must remain unchanged, even over a long period of time and even after a large number of operations of being wound on to and unwound from the winding drum. In the case of portions of different configurations on the pull element, such as for example holes, tooth arrangements and the like, they tend to be subjected to a high level of mechanical wear and thus enjoy only a limited service life.
In contrast thereto, determining the rotary movement of the winding drum by means of a rotary angle sensor which is operatively coupled to the rotary movement of the winding drum involves structure which is virtually wear-free and thus enjoys a very good overall service life.
An object of the present invention is to provide a pull element travel sensor of simplified mechanical structure, thereby to afford a pull element travel sensor of minimised axial length.
Another object of the present invention is to provide a pull element travel sensor capable of winding the pull element on the winding drum thereof without involving complicated mechanical components.
Still another object of the present invention is to provide a handling apparatus such as a lifting apparatus including a pull element travel sensor, capable of providing for precise vertical positioning of an object connected to the handling apparatus.
In accordance with the principles of the present invention, the foregoing and other objects are attained in a first aspect by a pull element travel sensor comprising a pull element winding drum and a biassing unit operatively connected thereto for biassing the winding drum in the direction of rotation thereof for winding the pull element on to the winding drum. A detection unit is operable to detect the length of the pull element which has been drawn off the winding drum. The pull element is wound on the winding drum in a single radial plane of the winding drum in a spiral configuration in a plurality of layers in mutually superposed relationship.
Further in accordance with the present invention, in a second aspect thereof, the foregoing and other objects are attained by a handling apparatus such as a lifting apparatus for lifting a load, comprising at least first and second lifting units and at least one travel sensor. Each lifting unit includes a pull element winding drum and a biassing unit operatively connected thereto for biassing the winding drum in the direction of rotation of the winding drum for winding the pull element on to the winding drum. A motor drives the winding drum at least in the winding-on direction. The lifting units are arranged with downwardly facing free ends of the pull elements. At least one of the lifting units is provided with a detection device for detecting the length of the pull element which has been drawn off the winding drum and all pull elements engage the same load.
In accordance with the principles of the present invention moreover in a third aspect the foregoing and other objects are attained by a pull element travel sensor comprising a winding drum for winding a band on an external winding surface of a winding cylinder portion of the winding drum, and a housing in which the winding drum is rotatably mounted. At least one braking magnet is arranged at a point non-rotatable with the winding drum in such a way that it acts brakingly against the direction of rotation in contact-less magnetically on the winding drum at an eccentric region of action thereof, the winding drum at least in the region includes electrically conductive material.
Further in accordance with the principles of the invention in a fourth aspect the foregoing and other objects are attained by a pull element travel sensor comprising a winding drum for winding a band on to an external winding surface of a winding cylinder portion of the winding drum, and a housing in which the winding drum is rotatably mounted. At least one braking magnet arranged eccentrically on the winding drum is operable to brakingly act on the housing in the direction of rotation in contact-less mode magnetically, the housing, at least in the region in which the magnet acts thereon, has electrically conductive material.
Additionally in accordance with the invention the foregoing and other objects in a fifth aspect are attained by a pull element travel sensor comprising a winding drum for winding a band on the external winding surface of a winding cylinder portion of the winding drum, and a housing in which the winding drum is rotatably supported. At least one holding magnet is arranged on the winding drum radially within the winding surface of the winding cylinder portion of the winding drum and is operable to hold the band radially inwardly to the winding surface, the band having magnetisable material.
In accordance still with the principles of the invention in a sixth aspect the foregoing and other objects are attained by a pull element travel sensor comprising a winding drum for winding a band on to an external winding surface of a winding cylinder portion of the winding drum, and a housing in which the winding drum is rotatably mounted. A sliding band extends around at least a part of the external periphery of the winding surface of the winding drum, which is at least in part movable with the band. The sliding band in particular comprises textile material, more especially a friction band or a band of other slidable, non-abrasive material. The band is passed around the winding surface at a small spacing in relation thereto or in rubbing relationship at the outside periphery of the winding of the winding band on the winding drum. The travel sensor has a device for adjusting the spacing or the contact pressure of the sliding band against the turns of winding band on the winding drum.
As will be seen in greater detail from preferred embodiments of the invention as described hereinafter, the invention in its first aspect affords a mechanical structural simplification by virtue of the fact that the pull element is wound on to the winding drum in a spiral configuration in a single radial plane and not in axially juxtaposed turns. The disadvantage that this apparently entails in terms of evaluation of the length of the winding element which is drawn off or wound on to the winding drum, more specifically that with increasing winding of the pull element on the winding drum the peripheral length of a single winding on the winding drum becomes progressively greater, is eliminated by a non-mechanical electronic device.
In order to determine the precise length of the pull element which has been drawn off the winding drum, the arrangement involves the use of an electronic evaluation system which does not associate a pull element length which is always the same, with a given angle of rotary movement of the winding drum, but also takes account of the instantaneous number of turns on the winding drum at a given moment.
A particularly simple configuration lies in an evaluation circuit which is capable of learning, referred to as a learning evaluation circuit, such that, before the pull element travel sensor is brought into operation, a single time, a defined length of the pull element is drawn off the winding drum, for example the known complete length of the pull element, and the angular segments of the winding drum or the rotary angle sensor, which have been covered in that procedure, are recorded by the electronic evaluation system. All smaller and possibly even larger draw-off lengths of the pull element are suitably extrapolated or interpolated, for example, arithmetically.
In a preferred embodiment, it is also possible to take into consideration the jump in winding diameter which occurs when winding the pull element on the winding drum, at a respective given rotational position which corresponds to the point at which the inner end of the wound-on pull element is fixed to the winding drum.
If the winding drum biassing unit is formed by conventional flat spiral springs, and if the detection unit for detecting the drawn-off length of the pull element is in the form of known rotary angle sensors, then, except for the simplified housing structure in the greatly simplified region at which the pull element passes into the housing of the travel sensor, all previously known components and possibly even the housing itself can continue to be used. Modifications are thus required only in terms of the electronic evaluation system.
Another option is that the detection unit directly ascertains the relative movement of the pull element with respect to a stationary point, for example the housing, with the detection unit being fixed to the housing for example in the proximity of the location at which the pull element passes into the housing.
For that purpose, the pull element, in the longitudinal direction thereof, must have distinguishable portions arranged at a defined spacing and the resolution of which also determines the measurement accuracy thereof. As measurement sensors of that kind are intended to implement operations for determining position, which are precise to a tenth or even a hundredth of a millimeter, it is generally more complicated and expensive to provide for such portion-wise distinguishability with individual portions which are only a few tenths or even a few hundredths of a millimeter in length over the entire length of the pull element, than on the basis of a single revolution of a rotary angle sensor. Preferably, the portions of the pull element are not mechanically distinguishable but are adapted to be electrically distinguishable, for example by being differently magnetised in portion-wise manner or by being of a metallic and non-metallic nature in a portion-wise manner or by virtue of being adapted to afford different optical reflective properties by means of suitable treatment methods.
In the latter case, the basic material selected may not be a metal band which otherwise forms a mechanically preferred embodiment of the pull element. In principle the pull element used, instead of an otherwise conventional cable as is used when the situation involves winding a single layer on a winding drum, is preferably a wide band which is as thin as possible in comparison to its width, in order to permit the band to be tidily wound in a spiral configuration in a single radial plane, so that in each turn it is repeatedly possible to achieve precisely the same winding length in each new winding operation.
In principle the pull element must be stable in the longitudinal direction, that is to say as far as possible it is not to be stretchable, while in the thickness direction it is to be of a thickness which is as far as possible always the same, and therefore cannot be expanded and/or compressed.
In addition however in principle the following disadvantages may occur in practical operation.
By virtue of the spring biassing effect on the winding drum, if for example an excessively low level of resistance is applied to the pull element when it is being wound on to the winding drum, the winding drum can reach very high speeds of rotation. As a consequence, at the end of the winding-on movement, the free end of the pull element hits against the entry to the housing with a high level of inertia, which can result in damage.
For the same reason, the pull element can also rise up away from the winding surface of the winding drum, that is, a radial spacing is formed between the pull element and the peripheral winding surface of the winding drum, due to the pull element being thrown radially outwardly away from the winding drum as it is being wound thereonto, and in that situation, in a worst-case scenario, an irregular winding of the pull element on the winding drum may result.
In consideration of the foregoing problems, and in regard to the further aspects of the invention as outlined above, involving a retardation or braking magnet on the housing or the winding drum, as will further become apparent from the description hereinafter of preferred embodiments of the invention, it is possible to limit the speed of rotary movement of the winding drum by the use of the contact-less magnetic braking assembly. The components involved in that structure are to comprise an electrically conductive material.
The braking moment can be generated between an eccentrically disposed region of the drum, preferably a region which is in the proximity of the external periphery thereof, and a stationary point which is thus non-rotatable with respect to the drum, for example a part of the housing of the drum.
As the pull element is wound on to the winding drum on the radially outwardly disposed peripheral surface thereof, the magnets are preferably oriented in a longitudinal direction, that is to say parallel to the axis of rotation of the winding drum, between two components which are adjacent to each other in that axial direction.
In such an arrangement, by virtue of the rotary movement of the winding drum, and irrespective of the spacing of the retardation or braking magnet relative to the component to be influenced thereby, an eddy current is first produced in the component which carries the retardation or braking magnet. The consequence of that eddy current is a magnetic field which is closed by way of the portion which is not in the magnetic field, thereby producing a braking moment.
Besides the strength of the magnet used, the braking action is determined to a very great extent by the spacing between the magnet and the component to be influenced thereby, and it is for that reason that this spacing should preferably be adjustable.
The respective component, which is to be subjected to the influence of the braking magnet, must include electrically conductive material, for example aluminum. When the magnet or magnets are arranged on the drum, that increases the inertial mass thereof, and that therefore affords the aspect of the invention which entails arranging the braking magnet or magnets at a stationary point, for example, on the housing. That would also remove the compulsion for the provision of at least a pair of braking magnets on the winding drum, which would give rise to a winding drum unbalance, and that arrangement also makes available more space for example for fitting a magnet holder for adjustability of the air gap.
A consideration which is in favour of arranging the magnet or magnets on the drum however is the possibility that the magnet or magnets can be used at the same time and thus as a functionally combined unit as a holding magnet or magnets.
In this respect, in this specification the term holding magnet is used to denote a magnet which is intended to prevent the pull element or band from climbing up or rising away from the winding surface formed by the external surface of the winding cylinder portion of the drum, insofar as the material of the pull element, which can be in the form of a band and which for this purpose is necessarily magnetisable, is drawn radially inwardly by magnetic force towards that winding surface. Accordingly holding magnets of that kind have to be disposed radially within the winding surface on the drum, and preferably therefore fixed on the inside of the winding cylinder which consists of a thin material, preferably in turn distributed over the periphery of the drum. In this case also, the recommendation is for the axis of magnetisation of the magnet or magnets to be oriented parallel to the axis of rotation of the drum, that is to say, it is recommended to provide a bar magnet, in particular a permanent magnet, which extends in the longitudinal direction of the drum.
In both cases the magnetic force can be increased by the provision of pole shoes or pole pieces, that is to say involving close contact of iron materials against at least one outside of the magnet, in order thereby to reduce the level of magnetic losses. Preferably, the arrangement does not involve any covering by a pole piece in that direction in which the field lines are required to pass into or out of the magnet freely. Therefore, primarily cup-shaped pole pieces are preferred, which in the case of the retardation or braking magnet are directed with their open side towards the component to be influenced by the magnetic effect and which, in the case of the holding magnet, are directed with their open side radially outwardly towards the pull element or band.
It is also possible to achieve an increase in the level of retardation or braking force by using rare earth magnets, that is to say magnets with components of samarium, cobalt, neodymium and/or boron. In particular rare earth magnets of that kind can be used to produce disk-shaped magnets whose magnetisation axis extends in parallel relationship to the thickness of the disk through the disk and/or which in that case can be magnetised differently in a sector-like configuration.
These type of disk-shaped magnets can be disposed in the constricted conditions in terms of space of pull element travel sensors, more easily than elongate bar magnets.
In particular such disk-shaped magnets can be accommodated in the end of screwthreaded pins or bolts as magnet holders which, in the component carrying them, can be moved by screwing closer towards or further away from the component to be influenced by the magnetic effect, in order thereby to provide for adjustability of the desired effect.
As an alternative and/or a supplement to the holding magnet or magnets, as indicated above, it is further possible in accordance with the invention to pass around the outside periphery of the movable winding drum a sliding band or belt, in particular a textile band or belt or a felt band or belt or more particularly a belt of plastic material such as PE, POM or PTFE, which when the pull element is wound correctly on the winding drum does not involve any contact with the wound turns of the pull element or bears against them without applying any force thereto, but which in contrast applies a force to the pull element if it climbs up on to a previous turn already formed on the winding drum.
Further objects, features and advantages of the invention will be apparent from the description hereinafter of preferred embodiments of the invention.